SUMMARY The long-term goal of this project is to investigate mechanisms underlying noise-induced cochlear synaptopathy and hidden hearing loss. Noise is a common risk factor for hearing loss. Recent studies have demonstrated that even a single episode of noise overexposure induces transient hearing loss, i.e., temporal threshold shift (TTS), noise-exposed animals could have no hair cell loss but have extensive spiral ganglion neuron (SG) and synapse degeneration. In particular, low spontaneous rate (LSR) auditory nerves and their synaptic connections with inner hair cells are preferentially lost. The noise-exposed animals and humans demonstrate normal hearing threshold and sensitivity (i.e., hidden hearing loss) in the early stage but will eventually exhibit other hearing disorders and hearing loss. Currently, the underlying mechanism for such cochlear synaptopathy remains unclear. Noise stimulates hair cell and neuron over-activation and increases K+ efflux that leads to increasing extracellular K+ concentration. It is well-established that high extracellular K+ can induce toxicity leading to second cell death in the brain following injury and stroke. We hypothesize that high, excess extracellular K+ following noise exposure can also cause SG synapse and neuron degeneration in the cochlea. We previously found that ATP purinergic P2X receptors in the cochlea are required for sinking K+ to re-enter into cells. A recent study also demonstrated that P2X2 receptors are necessary for the development of TTS. In addition, we found that P2X2 mutation can increase susceptibility to noise and induce hearing loss. These studies indicate that P2X receptors may have a critical role in noise-induced cochlear synaptopathy and hidden hearing loss. In this project, we will first test whether high extracellular K+ can cause SG synapse and neuron degeneration (Specific Aim 1, SA1). Then, we will identify and characterize P2X receptor expression in SG neurons, including LSR and HSR (high spontaneous rate) fiber synaptic endings, and test whether P2X receptors can mediate K+-sinking in the SG neurons. In SA3, we will test whether deficiency of P2X receptors can induce and exacerbate cochlear synaptopathy and hearing loss following noise exposure and high-K+ challenge. Completion of these studies will directly reveal the molecular mechanism underlying noise-induced cochlear synaptic degeneration and hidden hearing loss. These novel studies will also open a new therapeutic avenue for targeting noise-induced hearing loss and cochlear synaptopathy.